Joint cracks, common features in rocks, especially igneous ones, primarily form through two key mechanisms, as well as tectonic and weathering factors. These processes are:
Mechanisms of Joint Formation
Click to Expand Mechanisms of Joint Formation
<p>Joints and fractures in igneous rocks are produced in various ways at different times from their initial formation to their weathering.</p>
<p>While general tectonic stress and unloading can cause joints, the most common methods of joint creation in intrusive and extrusive rocks can be categorized into these two types:</p>
<ul>
<li><strong>Brittle flow</strong>: This occurs when the rock is subjected to stress that exceeds its strength, causing it to fracture. It is also known as fracturing.</li>
<li><strong>Cooling and contraction</strong>: As molten rock (magma or lava) cools, it contracts in volume. This contraction creates tension within the rock, leading to the formation of joints and fractures.</li>
</ul>Detailed Explanation
Brittle Flow (Fracturing)
- Rocks, though seemingly solid, can undergo deformation under stress. When the stress surpasses a rock's ability to deform plastically, it fractures.
- This brittle fracture is common in both intrusive and extrusive igneous rocks. Intrusive rocks cool slowly and can experience fracturing long after their formation and solidification. Extrusive rocks, on the other hand, will fracture from stress applied during or after cooling.
- Tectonic forces from movements deep within the Earth can apply pressure to rocks resulting in fractures, i.e. joints.
Cooling and Contraction
- Igneous rocks originate from molten material.
- When molten rock (magma or lava) cools, it transitions from a liquid to a solid state.
- During this cooling process, the rock's volume decreases, causing it to contract.
- This contraction produces tension within the rock mass.
- If the tensile stress exceeds the rock's strength, it results in fractures or joints.
- This mechanism is especially important in lava flows and intrusions, where large volumes of molten rock cool and solidify.
Additional Factors
While the above two processes are the main ones:
- Tectonic Deformation: Movement of the earth's crust causes stress that can result in fractures.
- Unloading and Weathering: Removal of the top layers of rock or weathering of the rock can lead to the formation of joints.
In Summary
| Formation Method | Description |
|---|---|
| Brittle Flow | Rock fractures when stress exceeds its strength, also called fracturing. |
| Cooling & Contraction | Volume reduction as molten rock cools, creating tension that causes fracturing. |
| Tectonic Deformation | Movement and stress from earth's crustal shifts. |
| Unloading & Weathering | Removal of surface layers and weathering of rock can cause joint formation. |
Understanding these mechanisms helps explain the various patterns of joints seen in igneous rock formations, from columnar jointing in lava flows to more irregular patterns in intrusive masses.
